The effects of La-Co co-substitution on the structural and magnetic properties of strontium M-type hexaferrites (SrM) were carefully investigated for the Sr_1−xLa_xFe_12−xCo_xO₁₉ (0.0 ≤ x ≤ 0.3) polycrystalline samples prepared by solid-state reaction in air. Without the use of sintering additives, La-Co co-substituted SrM single phases could be obtained from polycrystalline bulk samples by employing proper processing conditions. The lattice parameter a initially increases with increasing x from 0.0 to 0.1 but gradually decreases with increasing x to 0.20, and then remains almost unaltered up to x = 0.3; the lattice parameter c monotonously decreases with increasing x from 0.0 to 0.25, but it turns to an increase when x = 0.3. The reduction in c/a ratios and V_cell values with increasing x up to x = 0.25 are obviously attributable to the decreasing size effect resulting from La³⁺ substitution at the Sr²⁺ site, which surpasses the increasing size effect due to Co²⁺ and Fe²⁺ occupancy at the Fe³⁺ site. Meanwhile, with increasing x from 0.0 to 0.3, while the saturation magnetization (M_S) continuously decreases from 75.90 to 72.07 emu/g, the magnetic anisotropy field (H_a) increases from 20.1 to 24.7 kOe, leading to an increase in the intrinsic coercivity (H_ci) from 2.68 to 3.99 kOe. The gradual increase in H_a with x, probably caused by a gradual decrease in the crystallographic symmetry, is inversely proportional to the variation in the c/a ratios up to x = 0.25 as usual except when x = 0.3, of which deviation needs further study.

We carefully investigated the effects of La-Co co-substitution and magnetic field pressing (MFP) on the structural and magnetic properties of SrM hexaferrites. Samples composed of Sr_1−xLa_xFe_12−xCo_xO₁₉ were sintered at 1230 °C for 2 h in air with sintering additives composed of 0.7 wt% CaCO₃ and 0.7 wt% SiO₂. A single M-type phase was confirmed to exist up to x = 0.3. Rietveld refinement revealed a slight decrease in lattice parameter a and the unit cell volume (V_cell) with an increasing x, while parameter c showed a significant decrease. The saturation magnetization (M_s) values increased from 70.90 to 72.40 emu/g with an increasing x from 0.0 to 0.15 and then decreased to 71.38 emu/g with further increasing of x to 0.3, while the anisotropy field (H_a) increased from 17.7 to 25.9 kOe, leading to a continuous increase in the intrinsic coercivity (H_ci), from 3.52 to 5.00 kOe, respectively. Using the MFP process, the c-axis of M-type hexaferrite grains could be effectively aligned to the applied field direction, which significantly affected the microstructures and, thus, magnetic properties of samples. Unlike non-MFP samples, exhibiting a significant increase in the average grain size (d_avg) but almost unaltered average thickness (t_avg) with an increasing x from 0.0 to 0.3, MFP-processed samples exhibited almost unaltered d_avg values but a continuous decrease in t_avg. Consequently, the variation in remanent flux density (B_r) versus x followed that of M_s versus x and thus exhibited the highest B_r of 4.05 kG for x = 0.15, leading to the highest maximum energy product {(BH)_max} of 3.62 MGOe. With an increasing x from 0.0 to 0.3, the H_ci values continuously increased from 3.14 to 3.84 kOe mainly due to a continuous increase in H_a, although they were significantly lowered in comparison with those of non-MFP samples because of a large increase in B_r for a given composition x. A higher M_r/M_s ratio always resulted in a larger (BH)_max in our samples regardless of x. A careful comparison of the microstructures and magnetic properties between MFP and non-MFP samples provided valuable insights into a broad area of permanent magnet optimization.

A series of charge-balanced Si⁴⁺-M^1+,2+ (M = Mg²⁺, K⁺, Li⁺) substitution compositions with the chemical formulae of SrFe_12−2xSi_xMg_xO₁₉ (x = 0, 0.05, 0.1, 0.2), Sr_1−yFe_12−ySi_yK_yO₁₉ (y = 0.05, 0.1, 0.2), and SrFe_12−z(Si_0.6Li_0.6)_zO₁₉ (z = 0.05, 0.1, 0.2, 0.4, 0.6) were prepared using conventional ceramic processes. While the sole doping of Si with x = 0.1 (SrFe_12−xSi_xO₁₉ (x = 0.1)) causes a noticeable Fe₂O₃ phase formation, the co-doping of Si-Mg and Si-Li allow a single M-type phase formation with x up to x = 0.1 and z = 0.4, respectively. Notably, a 2.6% increase in the saturation magnetization was obtained for Si-Li-substituted SrM with z = 0.05—that is, SrFe_11.95Si_0.03Li_0.03O₁₉. Enhancement of the magnet performance can also be achieved when anisotropic permanent magnets are fabricated based on the substitution composition SrFe_11.95Si_0.03Li_0.03O₁₉. The remnant magnetic flux density was improved by 2.5% compared to that of the unsubstituted SrM (from 4207 to 4314 G). The maximum energy product (BH_max) value also increased from 4.24 to 4.46 M·G·Oe. The enhancement in permanent magnet performance is attributed to the increase in the M_S of SrM by the optimal Si-Li substitution. This is a promising result because enhanced permanent magnet performance is achieved with intrinsic magnetic property improvement.

Rare-earth permanent magnets, such as Nd₂Fe₁₄B, have been widely used in electric vehicle and wind turbine motors due to their high anisotropy field (H_a), saturation magnetization (M_s) and coercivity (H_c). Cerium (Ce) has gained attention as a potential alternative to neodymium (Nd) due to its high abundance and low cost. The relatively poor intrinsic magnetic properties of Ce magnets, however, remain a significant challenge for their industrial applications. In this study, the synthesis of Ce-based RE₂Fe₁₄B (2-14-1) phases was achieved by a modified reduction-diffusion (R-D) process using REFeO₃ (RE = Ce, Nd) as a precursor. The precursor was prepared by a solid-state reaction with CeO₂, Nd₂O₃, Fe₂O₃ and Fe powders, which is a much more suitable process for mass production and cost-effectiveness. Optimal composition and heat treatment conditions enabled the formation of single-phase Ce-based 2-14-1 particles. The as-synthesized single-phase Ce₂Fe₁₄B particles exhibited an M_s value of ~120 emu/g and an intrinsic coercivity (H_ci) value of ~85 Oe, which can be attributed to the large particle size as observed by FE-SEM.